The cylinder pressure of an internal combustion engine is measured by pressure transducers with associated electronic circuitry. The result of the measurement is available in the form of a current or voltage quantity which is proportional to pressure and by which the individual peak values are detected with electrical means. Thereupon, the mean value, as well as the difference between the lowest and the highest peak value within a number of N peak values, is determined from this defined number N of these values. This purely-electronic apparatus is suitable quite generally for the evaluation of periodic mechanical measuring quantities which have been converted to electrical quantities by various types of transducers.
The peak pressure of internal combustion engines may rise in the course of operation, and this may lead to overloading of various parts, in particular the bearings. This leads to fracture and failure of the engine. In order to be protected against such undesirable surprises, it is usual in particular in large Diesel engines, to measure the peak pressures from time to time and to correct them, if necessary. A correction is also necessary when these values are too low, in order to reduce the fuel consumption.
For a long time, mechanical peak pressure meters of relatively small size have been known which are portable and which are predominately used in connection with Diesel engines. All of these systems are based on a piston or valve plate which is displaceable in a cylinder member. The ignition pressure is guided into this cylinder, thus displacing the piston against a spring. Depending upon the model, the indication of the pressure is effected in various ways. In one known form of peak pressure meter, the piston displaces a writing pen on paper in the vertical direction. The length of the line gives the measure of the peak pressure. With horizontal advance of the paper, the pressure-time diagram of the cylinder pressure is recorded. In a further system, the spring pressure is adjusted until it is just so large that the peak pressure is insufficient to shift the valve plate. The spring bias is then detected as the measure for the peak pressure and is read off a scale.
All of these mechanical systems assume that an indication tube is present on the motor which permits the peak value measuring apparatus to be connected to the engine cylinder. This indicator tube is the cause of various difficulties which lead to measuring errors. The indicator tube constitutes a blind duct which is connected to the combustion space and is known to generate whistle oscillations in response to the rapid pressure rise in the combustion space. This causes a falsification of the pressure course and, in the peak value measuring apparatus, an erroneous indication of the peak values. Moreover, these indicator tubes are inclined to carbonize, this likewise leading to measuring errors.
Accordingly, piezoelectric pressure transducers are preferred which are built directly into the combustion space, flush with the wall thereof, whereby the undesirable influence of the blind duct is eliminated. Piezoelectric pressure transducers can be constructed to be very small and may be integrated in a spark plug. Thereby, it is possible to measure pressure courses in Otto engines without an indicator tube or indicator bore.
For an indication of cylinder pressure by means of piezoelectric pressure transducers heretofore, additionally an electrometer amplifier or charge amplifier was necessary in order to convert the pressure-proportional charge delivered by the transducer to a measuring voltage, as well as a light beam oscillograph for recording the pressure diagram. However, with such an arrangement, the cost is so large that the measurements are performed, for the most part, only by the manufacturers of the engines and do not come into practical use for servicing work by the user of the engine.
In the peak value measuring and evaluating apparatus described herein, a piezoelectric transducer is used. The electrical measuring signal produced by the transducer is not, however, recorded by a light beam oscillograph, but rather, the said apparatus measures the peak values of each pressure cycle during a defined number of N cycles and the mean value as well as the deviation of the lowest from the highest peak value is determined and indicated therefrom. Thus, the apparatus directly makes that evaluation which heretofore was obtained only in the office as a belated evaluation of the oscillograph diagram. The piezoelectric pressure measuring method permits only the measurement of relative pressures. Absolute pressures, such as measured by the described mechanical peak pressure measuring apparatus, cannot be handled thereby.
As in other arrangements, piezoelectric measuring chains, too, have the property that the signal zero point drifts away during the measurement. Admittedly, it is possible to balance the signal zero point at the beginning of the measurement with the engine at standstill in such a manner that, with the pressure zero at the pressure transducer, the indicator, measuring or register apparatus is likewise at zero. However, in the measuring operation with the engine running, the zero point is displaced in consequences of the heating of the transducer in the engine as well as by interferences charges caused by the charge amplifier or electrometer amplifier and finally by the dynamic transfer characteristic of the measuring system. When, after completed measurement, the pressure at the transducer is zero again, the signal zero point can then differ greatly from its original position at the beginning of the measurement. For a faultless measuring result, this zero point drift error must be taken into account continuously during the measurement. For assessing the course of combustion in an engine cylinder, it is insufficient to know the particularly-high peak value of a single combustion, such as detected by individual mechanical peak value meters. It is desirable to know the mean value of a certain number of peak pressures as well as the stray width thereof. This necessitates a measurement over a period of time during which the drift error may even assume impermissibly-large dimensions.
The development engineer on the test bed of the manufacturer can draw into its pressure-time diagram the zero line which has drifted away, can measure the peak pressures and average them as well as form the peak pressure stray by a graphic-calculatory method. However, this possibility is not available to a serviceman. Even if he could carry with him the apparatus, he would hardly have the time and the possibility to be able to perform this graphic evaluation. The apparatus described in accordance with the present invention, however, performs by itself this evaluation practically instantaneously. However, the use of this peak value measuring and evaluating apparatus is not limited only to piezoelectric transducers. It can also be used in conjunction with any other electro-mechanical transducer systems in which zero drift problems occur. Generally, the use is not limited to transducers; it extends to every situation where the demand is for the zero point related signal course of a periodic measurement voltage, where the measurement signal zero point varies.